Changes in v1.0.0-beta-13

Commits since v1.0.0-beta-12:

feat: make dmenu stdin reading non-blocking, add inputonly/I flag to hide everything but input, print out entered text in that case (ee16faa)
fix: hide preview initially (1398646)
fix: hardcode colors in default theme to avoid transparency caused by themes not setting gtk theme vars (c105db0)
fix: elephant submenus not working (b78ac4b)
fix: initial preview handle when items change (03ed9e3)
fix: async image loading to avoid blocking ui (0c84906)
minor: refactore image transformer func signature (468c046)